Heat treatable ti-al-nb-si alloy for gas turbine engine

ABSTRACT

Heat treatable titanium alloys of the Ti 3  Al type comprise 20 to 23 Al - 9 to 15 Nb-0.5 to 1.0 Si balance essentially T; (at %). These alloys exhibit a good balance of properties at room temperature and at high temperature (600° C. plus) especially when solution treated in the β field and artifically aged. Zr, V and Mo can be included in the alloys.

BACKGROUND OF THE INVENTION

This invention relates to titanium alloys based on or containing theordered intermetallic compound Ti₃ Al and having properties suitable forutilization in high temperature applications. The invention isparticularly, though not exclusively, directed to materials for use ascomponents in the compressor section of gas turbine engines.

Titanium based alloys have enjoyed significant usage as compressorsection materials because of their strength to weight advantage overalternative materials such as steels. However existing commercialtitanium alloys of the conventional titanium base type have limitedtemperature tolerance in terms of resistance to creep and resistance tooxidation. These limitations restrict the application of the establishedtitanium alloys to the lower pressure stages of the compressor wherecomponents are not subjected to temperatures significantly above 540° C.In the higher pressure stages of the compressor more refractorymaterials such as iron or nickel based superalloys are used despite theweight penalty they impose. There is a commercial drive towards the`all-titanium` compressor in order to save weight by elimination of ironor nickel based superalloy components. There is also a drive to increasethe compressor pressure ratio in order to improve overall engineefficiency and this would impose an increased temperature burden oncompressor section components.

DISCUSSION OF THE PRIOR ART

The established titanium alloys are based on a matrix consisting of oneor the other, or a mixture of the two, of those phases found in puretitanium. These phases are the α phase which is the lower temperaturephase end of hexagonal close-packed (hcp) structure and the β phasewhich is of body centred cubic (bcc) structure. The β phase is stablefrom the transus temperature of 882° C. up to the melting point.Alloying additions change the temperature at which the α to β transitionoccurs. Some elements lower the β transus temperature and these aretermed β stabilizers. Others which raise the β transus temperature aretermed α stabilizers. The alloys are usually catergorised having regardto their predominant microstructure at room temperature and to thenature and proportions of the alloying ingredients, into the followinggroups: α-type alloys; β-type alloys and α+β type alloys. The α groupalso includes those alloys termed near-α alloys.

A digression is made here to explain that the atomic percent system isused in the main in this document in defining and describing theinvention, compositions given in these terms being designated "at %". Incommercial practice it is conventional to specify compositions in theweight percent system and that system is retained here when makingreference to prior art alloys specified by weight in the sourcedocument. Compositions specified by weight are designated "wt %".

IMI 829 is a commercial alloy which is representative of the best ofestablished gas turbine engine titanium alloys in terms of creepstrength and oxidation resistance in regard to high temperatureproperties (IMI 829 is a trade designation of IMI Titanium). This near-αalloy has a nominal composition Ti-5.5Al-3.5Sn-3Zr-1Nb-0.25Mo-0.3Si (at%). The properties of this alloy are used as one baseline for comparisonat various points in this specification. It is limited by hightemperature oxidation and its deleterious effect on fatigue propertiesto applications not requiring exposure to temperatures of 550° C. andabove.

One of the alloying elements used in the established titanium-basealloys is aluminium, which is an α stabilizer. If aluminium is added totitanium in suitable proportion on ordered intermetallic compound Ti₃ Alis formed. This is designated the α₂ phase and it has a ordered hcpstructure. In the established alloys the aluminium content is restrictedby reference to an empirical rule to a level beneath that at which theα₂ phase starts to occur because this phase is regarded as embrittlinghaving regard to the ductility etc exhibited by the matrix material.However the properties of Ti₃ Al are such that it has attractedattention for some years as the possible base for a class of titaniumalloy having improved high temperature properties. The α₂ phase is knownto have particularly high stiffness combined with good creep resistanceand oxidation resistance. Aluminium is less dense than titanium so ahigh aluminium content is attractive in its own right for the consequentreduction in density. However, although there are many references in thetechnical literature to research into α₂ based alloy systems only onesuch alloy is known to have been commercialised to any degree and thisis produced by Timet Corporations (USA). Further reference is made tothis alloy later in this specification. In general the other α₂ alloyshave suffered from lack of ductility at low temperatures (ambient andabove) and have been of relatively high density compared withconventional titanium alloys.

Early work in the field of TI₃ Al based alloys was documented byMcAndrews et al in several reports issued in the 1960s. These alloyswere based on the Ti--Al--Nb system and tests were performed on theternary alloy and alloys with additions of Hf, Zr, C and B. The testedalloys cover Al contents of 7.5 to 17.5 wt % and Nb contents of 15 to 35wt % but not all combinations of each. The reports concluded that alloyswith high Nb and Al contents incorporating Hf and Zr showed the mostpromise.

In U.S. Pat. No. 3,411,901 (GB 1041701) there is disclosed Ti-basedalloys comprising 10 to 30 wt % Al and Nb where the level of Nb is 8/7of the Al level (by weight) plus or minus 5%. Si (up to 2 wt %) isdisclosed as a useful addition for the promotion of high temperaturestrength and oxidation resistance. Small quantities of Hf, Zr or Sncould be included for improvement of workability and high temperaturestrength. In the patent specifications the only comment given regardingthe microstructure of these alloys is the comment given in the USdocument but not the British one that the alloys are of the α-β type.These patent specifications provide only a little information regardingthe properties achieved by the alloys within the claimed range as far asis known by us these alloys have not found any degree of commercialacceptance, if indeed they have been produced on a commercial scale.

In GB 2060693A (United Technologies Corporation) there is disclosed arange of TI₃ Al based alloys. The range claimed as the invention is Tibase--24 to 27 Al--11 to 16 Nb (at %) and the preferred range is Tibase--24.5 to 26 Al--12 to 15 Nb (at %). These compositions whenexpressed in weight percent terms approximate to the following: broadrange Ti base--13.5 to 14.7 Al--21.4 to 30 Nb; preferred range Tibase--13.7 to 14.5Al--23.2 to 28.3 Nb. There are two comparisoncompositions of lower aluminium content disclosed these being Ti-22Al--10 Nb and Ti--22 Al--5 Nb (both at %). Significant importance isattached to the aluminium content in the document. It is stated that "Itis found that ductility and creep strength change inversely to eachother over a very narrow range of aluminium content, thus, the aluminiumcontent is very critical" . The 24 at % minimum figure for aluminiumlevel is based on a belief that at least this level is required tosecure a satisfactory creep strength (in the light of the trend datawithin the claimed range, and the poor properties of the 22 at %aluminium alloys) despite the noted adverse effect of increasingaluminium content on room temperature properties. The upper aluminiumlimit is fixed by the minimum level of room temperature ductility whichmay be tolerated and by the niobium level. The niobium range is limitedat the upper end by density considerations and is limited at the lowerend by the minimum level of room temperature ductility which may betolerated.

Within the claimed range of alloys in GB 2060693A there are six alloyexamples documenting the basic alloy--ie that without other ingredientsseen to be significant. The properties of these are documented in Table2 on page four of the referenced document in terms of tensile elongationat room temperature and creep rupture life when tested at 650° C. undera stress of 380 MPa. The listed compositions and properties of these keyalloys are reproduced below:

Ti-24 Al-11 Nb (at %)--elongation 4.0% creep life 20 hours

Ti-24 Al-11 Nb (at %)--elongation 3.0% creep life 65 hours

+ undisclosed Si level

Ti-25 Al-15 Nb (at %)--elongation 3.0% creep life 130 hours

Ti-26 Al-11 Nb (at %)--elongation 1.5% creep life 80 hours

Ti-26 Al-12 Nb (at %)--elongation 1.4% creep life 143 hours

Ti-27 Al-13 Nb (at %)--elongation 1.0 creep life 21 hours.

These alloys covered above were tested in a β phase solution treatedcondition without aging, and in consequence the results achieved interms of tensile elongation may be somewhat optimistic because generallyan aging treatment is likely to be required in order to secure asatisfactory level of tensile strength and to convey metallurgicalstability for use at the service temperature. It would be expected thatan artificial aging treatment or alternatively aging in service wouldreduce the ductility with respect to the pre aged material and our owntest of an alloy from within the above composition range when heattreated and aged bears out this expectation-see results given later. Itis noticable also that no tensile strength or yield data is given forthese unaged alloys.

GB 2060693A also discloses some additional ingredients. Vanadium is theingredient seen as most beneficial and an alloy having vanadium inlevels up to 4 at % in partial substitution for niobium is claimed.Other ingredients mentioned are Si, C, B (all in substitution for Ti)Mo, W (both in substitution for Nb) and Si, In (both in substitution forAl). These additional ingredients are mentioned as ingredients includedin prior art alloys which might have benefit in the claimed alloy. Eventhough one silicon containing alloy had been tested it had not been seento yield any benefit worthy of mention although the possibility that itcould have benefit was not rule out.

It was mentioned earlier that an α₂ based alloy is produced by TimetCorporation (USA). The position regarding the unavailability of thisalloy or alloys is uncertain and it may be unavailable outside the USA.Little property data has been disclosed and even the composition is notcertain. Brief press references appear to indicate that the alloy inquestion is Ti-24 Al-11 Nb (at %) and if this is correct it would appearto be an alloy made in accordance with the United Technologies patent.The composition Ti-24 Al-11 Nb has been used by us as a basis forcomparison for the alloy we claim.

OBJECT AND SUMMARY OF THE INVENTION

It is the object of this invention to provide a titanium alloy capableof extending the field of usefulness of such alloys (having regard tothe established conventional alloys) to above 600° C. in gas turbinecompressor sections and the like, and to provide such an alloy as hassuperior properties to those of prior art alloys based on Ti₃ Al and thelike. To be useful as a compressor alloy, the alloy must exhibit goodstrength, oxidation resistance and creep strength at the temperatures inquestion (600° C. and above). A viable Ti₃ Al alloy must exhibit theseproperties and also have sufficient ductility at room temperature afterforging to permit further processing. The claimed alloy can withappropriate preparation be tailored to yield superior high temperaturestrength and creep life for a given level of room temperature ductilitythan the alloys disclosed in the United Technologies patent (as (asevidenced by the data disclosed in the patent specification and our owntrials on Ti-24 Al-11 Nb).

The improvements achieved in the claimed alloy must be seen asunexpected, at least insofar as the United Technologies patent isconcerned, because the composition claimed flouts the firm guidancegiven in the patent specification regarding aluminium content, andrelies on silicon as a beneficial and necessary ingredient when nosignificant value had been given to this ingredient in the priordocument.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention is a heat treatable titanium alloy which is suitable foruse as components in the compressor section of a gas turbine engine andwhich is based on or contains the intermetallic phase Ti₃ Al, having acomposition within the range stated below in atomic proportions:

20 to 23% aluminium

9 to 15% niobium

0.5 to 1.0% silicon

0 to 3% zirconium

0 to 3% vanadium

0 to 3% molybdenum

balance essentially titanium;

and wherein there is not more than 5% in total of ingredients from thegroup consisting of zirconium, vanadium and molybdenum. It is notessential to include in the alloy any ingredient from theabove-mentioned zirconium, vanadium, molybdenum group as alloys havingsuperior properties to the prior art alloys can be produced from thebasic quaternary alloy of Ti-20 to 23 Al 9 to 15 Nb-0.5 to 1.0 Si whensuitably heat treated and aged.

It has been found that a niobium content of around 11 at % gives bestproperties with regard to the balance between creep rupture life androom temperature ductility. The niobium level appears to be moreimportant than aluminium level, in this regard, within the boundaries ofthe overall range claimed. Accordingly a preferred alloy range comprisesnominally 11% Nb with 20 to 23% Al, 0.5 to 1.0% Si and balanceessentially Ti.

The silicon which is an essential feature of the claimed alloy makes asignificant contribution to the properties of the alloy. The optimumsilicon level may vary from composition to composition within the bandclaimed and may also depend upon the precise balance of propertiesrequired of the alloy. It has been found that in general 0.9 Si yieldsbetter properties than 0.5 Si. A high silicon content is consideredundesirable in prior art alloys of the conventional variety so we deemit wise to limit the silicon content to 1.0% maximum in the claimedalloy and a preferred silicon range is 0.8 to 1.0 at %.

A preferred alloy comprising Ti-23Al-11Nb-0.9Si (at%) has been used asthe basis for testing the effectiveness of additional ingredients fromthe zirconium, vanadium, molybdenum group. An alloy with 2 at% Zrsubstituted for Nb yielded an improved combination of room temperaturestrength and ductility with creep rupture life. 2 at% V was alsobeneficial when introduced at the expense of Nb but it was lesseffective when introduced in substitution for Ti. An alloy comprisingTi-23Al-11Nb-0.9Si-1.0Mo which has been tested only in the `as forged`condition also yielded an improved combination of properties over thebase alloy in the same condition. A limit of 3 at% for each of theseadditional ingredients individually and a limit of 5 at% in total ofthese is deemed to be advisable in order to avoid overstepping theboundary of utility.

The properties of the claimed alloys and the methods for preparing andheat treating it are documented below with reference to severalexemplary compositions. Reference is made also to some comparisoncompositions outside the claimed range but not within the state of theart as far as is known. Two prior art compositions are documented alsofor comparison purposes these being:

a. IMI 829, as a representative of established conventional alloys, and

b. Ti-24Al-11Nb (at %), for assessment of the properties of the prior`commercial` Ti₃ Al alloy of Timet Corporation (USA)

All of the alloy samples produced and tested were prepared as 200 gbuttons by vacuum arc melting. After solidification and cooling from thefirst melt the buttons were turned and remelted (by the vacuum arcprocess) for improved homegeneity. These buttons were then isothermallyforged at 1000° C. to half original thickness at a strain rate of0.001/sec. These forged pieces were divided into several portions. Someportions were machined to yield tensile test and creep test specimens inthe as forged condition. Other portions were subjected to individualheat treatments before being machined to test specimen configuration.

The quaternary compositions investigated and the designations given toeach of these are detailed in Table 1 below. Two ternary Ti-Al-Nb alloysand IMI 829 are listed also.

                  TABLE 1                                                         ______________________________________                                        Alloy compositions (at %) - all have Ti as balance                            AL        Nb     Si         Alloy designation                                 ______________________________________                                        20        11     0.5        5F                                                20        11     0.9        5A                                                20        13     0.5        8A                                                20        15     0.9        4A                                                23        11     0.9        7A                                                23        15     0.5        9A                                                Comparison Alloys                                                             17        15     0.9        C1A                                               18        13     0.9        C6A                                               19        10     0.9        C2A                                               20        11     0          C5G                                               21         8     0.9        C3A                                               24        11     0          C12A                                              ______________________________________                                    

A variety of alloy conditions with regard to post-forging treatmentshave been investigated. These are documented in Table 2 below.

                  TABLE 2                                                         ______________________________________                                                                 Condition                                            Alloy Condition          Designation                                          ______________________________________                                        As forged (naturally cooled)                                                                           A                                                    Aged for 24 hours under vacuum at 800° C.                                                       B                                                    then fast gas cooled                                                          Solution treated for 1 hour under vacuum                                                               C                                                    at a temperature in the β field then fast                                gas cooled then aged for 24 hours at 700° C.                           under vacuum and again fast gas cooled                                        As C save that aged for 2 hours at 625° C.                                                      .sub. D.sub.1                                        As C save that aged for 2 hours at 700° C.                                                      .sub. D.sub.2                                        Solution treated for 1 hour at a temperature                                                           E                                                    in the α and β field                                               Solution treated for 1 hour at a temperature                                                           .sub. F.sub.1                                        in the α and β field then aged for 2 hours at                      625° C. then naturally cooled                                          As F.sub.1 save that aging temperature is 700° C.                                               .sub. F.sub.2                                        ______________________________________                                         NOTE                                                                          1. All fast gas cooling is by argon and at a rate of approximately            6° C./sec.                                                             2. In treatments E, F.sub.1  and F.sub.2 the specimens were treated in an     evacuated then argon filled quartz encapsulation in order to avoid oxygen     contamination in the natural cooling phase.                              

The β transus temperature was determined for each of the keypoint alloysby a conventional differential thermal analysis technique. The βsolution-treated specimens were solution treated at a temperature abovethe β transus. The solution treatment temperature varied from 1050° C.to 1125° C. depending upon composition. The α and β solution treatedspecimens were solution treated at a temperature below the β transus.The solution treatment temperature for these specimens was in the range900° C. to 1050° C. depending on composition.

It has been found that the properties of the claimed alloys, as withother Ti₃ Al alloys, are considerably influenced by the alloyconditioning. This variation in properties is documented with referenceto alloys 5A and 7A in Table 3 below. The property measurements used inTable 3 and the later tables are: tensile elongation at room temperature(nominally 20° C.) as a measure of ductility at this temperature,tensile strength at room temperature, and creep rupture life when creeptested in air at 625° C. under a stress of 250 MPa. The creep rupturetest was discontinued at 1000 hours for those specimens still intact atthis point.

For certain alloys the tensile elongation and tensile strength at 650°C. are also given in the tables.

                                      TABLE 3                                     __________________________________________________________________________                                      Creep Rupture                                         Tensile Strength (MPa)                                                                    Tensile Elongation (%)                                                                    Life                                        Alloy                                                                             Condition                                                                           at 20° C.                                                                    at 650° C.                                                                   at 20° C.                                                                    at 650° C.                                                                   (hours)                                     __________________________________________________________________________    5A  A     915   625   8.7     46.5                                                                              73.7 @ 150 MPa                                  B     767         3.6          7.4                                            C     730         0           215.8                                           .sub. D.sub.1                                                                       1125        2.0         245.9                                           .sub. D.sub.2                                                                       866         0           135.3                                           E     1069        6.3         239.4                                           .sub. F.sub.1                                                                       1222        1.9         299.3                                           .sub. F.sub.2                                                                       815         0           225.1                                       7A  A     762   475   3     25     98.4                                           B     --    --    --    --    264                                             C     536   --    0     --    >1000                                           .sub. D.sub.1                                                                       804   --    1.1   --    >1000                                           .sub. D.sub.2                                                                       1206  --    0.1   --    389.5                                           E     801   --    5.2   --    134.9                                           .sub. F.sub.1                                                                       823   --    1.9   --    313.1                                       __________________________________________________________________________

In general it has been found that the alloy condition designated D₁yields the most consistently good results. That is not to say it is thebest for all alloys, merely that it is a suitable basis on which tocompare the relative properties of the alloys within the claimed rangeand those alloys outside the claimed range. Table 4 below gives acomparison of principal properties for the claimed alloys and thecomparison alloys.

                                      TABLE 4                                     __________________________________________________________________________                    Tensile Tensile                                                                              Creep Rupture                                                  Strength                                                                              Elongation                                                                           Life                                           Alloy                                                                             Alloy Composition                                                                         (MPa) @ 20° C.                                                                 % @ 20° C.                                                                    (hours)                                        __________________________________________________________________________    4A  Ti--20Al--15Nb--0.9Si                                                                     1008    4.9    307.4                                          5A  Ti--20Al--11Nb--0.9Si                                                                     1125    2.0    245.9                                          5F  Ti--20Al--11Nb--0.5Si                                                                     1191    0.6    217.2                                          8A  Ti--20Al--13Nb--0.5Si                                                                     828     2.8    154.8                                          7A  Ti--23Al--11Nb--0.9Si                                                                     804     1.1    >1000                                          9A  Ti--23Al--15Nb--0.5Si                                                                     798     1.5    506.8                                          Comparison alloys                                                             C1A Ti--17Al--15Nb--0.9Si                                                                     1350    0.3     68.2                                          C2A Ti--19Al--10Nb--0.9Si                                                                     982     2.6     85.2                                          C3A Ti--21Al--8Nb--0.9Si                                                                      789     1.3    219.3                                          C5G Ti--10Al--11Nb                                                                            1142    0.5    180.1                                          C6A Ti--18Al--13Nb--0.9Si                                                                     1150    1.0    180.0                                          IMI829                                                                            Ti--5.5Al--3.5Sn--3Zr                                                                     950     9.0    114.2                                              0.25Mo--0.3Sr                                                             C12A                                                                              Ti--24Al--11Nb                                                                            728     0.0    576.7                                          __________________________________________________________________________

All the alloys within the claimed range have a useful combination of thethree properties documented in Table 4. They all have significantlysuperior creep rupture life than the conventional IMI 829 alloy and ausuable level of room temperature tensile elongation though as would beexpected this is not a comparable level to the conventional alloy. Thebalance of tensile elongation and creep rupture life for all thosealloys in the claimed range is superior to the alloys of the Ti₃ Al typelying outside the claimed range including the commercialisedTi-24Al-11Nb composition which in the D₁ condition has no tensileelongation although good creep rupture life. Tensile strength at roomtemperature is good for all alloys in the claimed range in thiscondition. For some alloys there is a considerable benefit in thisregard over the conventional IMI 829 alloy. A more comprehensivetabulation of properties for the principal alloys in the claimed rangeand comparison alloys, is given in Table 5 below.

                                      TABLE 5                                     __________________________________________________________________________                                      Creep Rupture                                         Tensile Strength (MPa)                                                                    Tensile Elongation (%)                                                                    Life                                        Alloy                                                                             Condition                                                                           at 20° C.                                                                    at 650° C.                                                                   at 20° C.                                                                    at 650° C.                                                                   (hours)                                     __________________________________________________________________________    4A  A     814   542   6.5   36.9   31.7                                           B     751         11.7         33.2                                           C     750         0           267.7                                           .sub. D.sub.1                                                                       1008        4.9         307.4                                           .sub. D.sub.2                                                                       1265        0.3         340.8                                           E     914         4.6          42.6                                           .sub. F.sub.1                                                                       977         3.6          40.5                                           .sub. F.sub.2                                                                       942         3.8          59.1                                       5A  A     915   625   8.7   46.5   73.7                                           B     767         3.6          7.4                                            C     730         0           215.8                                           .sub. D.sub.1                                                                       1125        2.0         245.9                                           .sub. D.sub.2                                                                       866         0           135.3                                           E     1069        6.3         239.4                                           .sub. F.sub.1                                                                       1222        1.9         299.3                                           .sub. F.sub.2                                                                       815         0           225.1                                       5F  A     879         9.8          6.0                                            .sub. D.sub.1                                                                       1191        0.6         217.2                                           .sub. F.sub. 1                                                                      1178        4.2          71.5                                       7A  A     762   475   3.0   25.0   98.4                                           B                             264.0                                           C     536         0           >1000                                           .sub. D.sub.1                                                                       804         1.1         >1000                                           .sub. D.sub.2                                                                       1206        0.1         389.5                                           E     801         5.2         134.9                                           .sub. F.sub.1                                                                       823         1.9         313.1                                       8A  A     888         13.4         17.4                                           .sub. D.sub.1                                                                       828         2.8         154.8                                           .sub. F.sub.1                                                                       1015        3.9          14.6                                       9A  A     874         7.2          87.5                                           .sub. D.sub.1                                                                       798         1.5         506.8                                           .sub. F.sub.1                                                                       902         1.0          39.6                                       Comparison Alloys                                                             C1A A     804   561   15.7  25.2  4.2 @ 300 MPa                                   B     760         17.6         3.7                                            C     871         0            99.3                                           .sub. D.sub.1                                                                       1350        0.3          68.2                                           .sub. D.sub.2                                                                       921         0           174.2                                           E     1084        2.4          39.6                                           .sub. F.sub.1                                                                       1194        2.5          37.2                                           .sub. F.sub.2                                                                       1168        3.3          14.8                                       C2A A     797   350   6.0   42.5   1.0                                            B     808         8.4          10.4                                           C     671         0            44.4                                           .sub. D.sub.1                                                                       982         2.6          85.2                                           .sub. D.sub.2                                                                       1061        0            19.0                                           E     1282        0            86.2                                           .sub. F.sub.1                                                                       1070        4.1          50.3                                       C3A A     887   453   4.1   45.2   20.0                                           B     809         14.4         35.0                                           C     673         0            76.6                                           .sub. D.sub.1                                                                       789         1.3         219.3                                           .sub. D.sub.2                                                                       1303        0.7         218.9                                           E     1003        1.1          73.2                                           .sub. F.sub.1                                                                       913         --           66.7                                           .sub. F.sub.2                                                                       1084        1.8         177.5                                       C5G A     874         8.0          2.7                                            .sub. D.sub.1                                                                       1142        0.5         180.1                                           .sub. F.sub.1                                                                       1249        0.6          92.9                                       C6A A     780   449   6.2   32.9   8.9                                            B     744         6.2          4.2                                            C     512         0           128.3                                           .sub. D.sub.1                                                                       1150        1.0         180.0                                           .sub. D.sub.2                                                                       1105        0            89.9                                           E     1114        4.1          62.3                                           .sub. F.sub.1                                                                       1150        1.9          30.6                                       C12A                                                                              A     824         3.1         267.3                                           .sub. D.sub.1                                                                       728         0           567.7                                       __________________________________________________________________________

The correlation of properties to composition for the claimed alloys maybe appreciated more readily by reference to Tables 6, 7 and 8 belowwhich show properties against varying aluminium, niobium and siliconlevels respectively for alloyes in the D1condition.

                                      TABLE 6                                     __________________________________________________________________________    Correlation of properties with regard to aluminium content                                    Tensile Tensile                                                                              Creep Rupture                                                  Strength                                                                              Elongation                                                                           Life                                           Alloy                                                                             Alloy Composition                                                                         (MPa) @ 20° C.                                                                 % @ 20° C.                                                                    (hours)                                        __________________________________________________________________________    C1A Ti--17Al--15Nb--0.9Si                                                                     1350    0.3     68.2                                          4A  Ti--20Al--15Nb--0.9Si                                                                     1008    4.9    307.4                                          9A  Ti--23Al--15Nb--0.5Si                                                                      798    1.5    506.8                                          C11B                                                                              Ti--17Al--11Nb--0.9Si                                                                     1195    0.5     64.5                                          5A  Ti--20Al--11Nb--0.9Si                                                                     1125    2.0    245.9                                          7A  Ti--23Al--11Nb--09Si                                                                       804    1.1    >1000                                          C15A                                                                              Ti--17Al--8Nb--0.9Si                                                                      1112    1.0    124.6                                          C3A Ti--21Al--8Nb--0.9Si                                                                       789    1.3    219.3                                          C14A                                                                              Ti--23Al--8Nb--0.9Si                                                                       699    1.7    164.9                                          __________________________________________________________________________

                                      TABLE 7                                     __________________________________________________________________________    Correlation of properties with respect to niobium content                                     Tensile Tensile                                                                              Creep Rupture                                                  Strength                                                                              Elongation                                                                           Life                                           Alloy                                                                             Alloy Composition                                                                         (MPa) @ 20° C.                                                                 % @ 20° C.                                                                    (hours)                                        __________________________________________________________________________    C14A                                                                              Ti--23Al--8Nb--0.9Si                                                                       699    1.7    164.9                                          7A  Ti--23Al--11Nb--0.9Si                                                                      804    1.1    >1000                                          9A  TI--23Al--15Nb--0.5Si                                                                      798    1.5    506.8                                          C3A Ti--21Al--8Nb--0.9Si                                                                       789    1.3    219.3                                          5A  Ti--20Al--11Nb--0.9Si                                                                     1125    2.0    245.9                                          4A  Ti--20Al--15Nb--0.9Si                                                                     1008    4.9    307.4                                          C15A                                                                              Ti--17Al--8Nb--0.9Si                                                                      1112    1.0    124.6                                          C11B                                                                              Ti--17Al--11Nb--0.9Si                                                                     1195    0.5     64.5                                          C1A Ti--17Al--15Nb--0.9Si                                                                     1350    0.3     68.2                                          __________________________________________________________________________

                                      TABLE 8                                     __________________________________________________________________________    Correlation of properties with respect to silicon content                                     Tensile        Creep Rupture                                                  Strength                                                                              Elongation                                                                           Life                                           Alloy Composition                                                                             (MPa) @ 20° C.                                                                 % @ 20° C.                                                                    (hours) at 625° C.                      __________________________________________________________________________    5A  Ti--20Al--11Nb--0.9Si                                                                     1125    2.0    245.9                                          5F  Ti--20Al--11Nb--0.5Si                                                                     1191    0.6    217.2                                          5G  Ti--20Al--11Nb                                                                            1142    0.5    180.1                                          C11B                                                                              Ti--17Al--11Nb--0.9Si                                                                     1195    0.5     64.5                                          C11A                                                                              Ti--17Al--11Nb--0.5Si                                                                      985    --      71.9                                          __________________________________________________________________________

The beneficial effect of silicon at the higher level examined isimmediately apparent from Table 8. The United Technologies patent (GB2060693) does not predict this effect. Indeed FIG. 3 in that documentwould seem to indicate that silicon lowers room temperature elongation.We have found that silicon raises both room temperature ductility andcreep rupture life without detriment to tensile strength. With thisbeneficial effect from Si secured at lower aluminium levels thanpreviously supposed this yields a tangible benefit of significantlyimproved room temperature tensile elongation with respect to the priorart Ti₃ Al alloy Ti-24Al-11Nb when tested under identical conditions.

The characteristics of the claimed alloys with regard to oxidationresistance are documented in Table 9 below. The alloys were tested in acyclic oxidation test of 100 hours duration in air at 700° C. Once every25 hours the test specimens were removed from the furnace, naturallycooled to room temperature, then replaced in the hot furnace. The degreeof oxidation penetration was determined through a microhardness traverseof a section of the tested specimens by virtue of the hardeningconsequent upon oxidation.

                  TABLE 9                                                         ______________________________________                                        Alloy     Condition Depth of hardening (μm)                                ______________________________________                                        5A        A         60                                                                  .sub. D.sub.1                                                                           60                                                                  .sub. F.sub.1                                                                           70                                                        7A        A         55                                                                  .sub. D.sub.1                                                                           75                                                                  .sub. F.sub.1                                                                           65                                                        IMI 829   .sub. D.sub.1                                                                           150                                                       C1A       .sub. D.sub.1                                                                           100                                                       ______________________________________                                    

It will be seen that the two examples of the claimed alloy showconsiderable reduction in the degree of oxidation penetration withrespect to the conventional titanium alloy IMI 829, and seen also thatthey are significantly better in this regard to the composition Ti₃ Alalloy CIA having a composition outside the claimed range.

The effect of various additions to the claimed quaternary alloy havebeen investigate using alloy 7A (Ti-23Al-11Nb-0.9Si at %) as a basis forcomparison. Alloy specimens to various compositions of interest wereprepared using the procedure previously described and subjected to thesame tests as used for the previous materials. Properties of thesemodified alloys and the baseline alloy 7A are given in Table 10 below.

                                      TABLE 10                                    __________________________________________________________________________                            Tensile Tensile                                                                              Creep Rupture                          Alloy                   Strength                                                                              Elongation                                                                           Life                                   Designation                                                                         Alloy Composition                                                                         Condition                                                                           (MPa) @ 20° C.                                                                 % @ 20° C.                                                                    (hours)                                __________________________________________________________________________    7A    Ti--23Al--11Nb--0.9Si                                                                     A     762     3.0    98.4                                                     C     536     0      >1000                                                    .sub. D.sub.1                                                                       804     1.1    >1000                                                    .sub. F.sub.1                                                                       823     1.9    313.1                                  7B    Ti--23Al--9Nb--0.5Si                                                                      A     755     2.1    50.1                                         2Zr         .sub. D.sub.1                                                                       840     1.9    >1000                                                    .sub. F.sub.1                                                                       840     1.0    208.3                                  7C    Ti--23Al--11Nb--0.5Si                                                                     A     804     7.5    60.4                                         2V          .sub. D.sub.1                                                                       893     2.8    747.3                                                    .sub. F.sub.1                                                                       1015    1.5    396.6                                  7D    Ti--23Al--9Nb--0.5Si                                                                      A     746     3.5    58.0                                         --2V        .sub. D.sub.1                                                                       808     2.7    >1000                                                    .sub. F.sub.1                                                                       845     2.8    275.0                                  7I    Ti--23Al--9Nb--0.9Si                                                                      A     1005    1.0    182.1                                        --2Mo                                                                   7J    Ti--23Al--11Nb--0.9Si                                                                     A     888     3.9    125.4                                        --1Mo                                                                   __________________________________________________________________________

The alloy 7B with 2 at % Zr substituted for Nb, has in the D₁ conditionimproved tensile strength and tensile elongation at room temperatureover the baseline alloy and comparable creep rupture life. Alloy 7D with2 at % V substituted for Nb, has in the D₁ conditions even highertensile elongation with comparable strength and creep rupture life tothe base line alloy.

The Mo-containing alloy 7J shows the best properties of all in the `asforged` A condition. This alloy has not yet been tested in otherconditions.

What is claimed is:
 1. A heat-treatable titanium alloy which is suitablefor use as components in the compressor section of a gas turbine engineand which is based on or contains the intermetallic phase Ti₃ Al,consisting essentially of the following constituents in atomicproportions:20 to 23% aluminum 9 to 15% niobium 0.5 to 1.0% silicon 0 to3% zirconium 0 to 3% vanadium 0 to 3% molybdenum balance essentiallytitanium;and wherein the proportion of optional constituents from thegroup consisting of zirconium, vanadium and molybdenum, when two or moreare present in combination, is up to 5 atomic percent.
 2. A titaniumalloy as claimed in claim 1 having a composition within the range statedbelow in atomic proportions:20 to 23% aluminium 9 to 15% niobium 0.5 to1.0 silicon balance essentially titanium.
 3. A titanium alloy as claimedin claim 1 comprising 0.8 to 1.0 atomic percent of silicon.
 4. Atitanium alloy as claimed in claim 1 consisting essentially of thefollowing ingredients in the atomic proportions below-stated:aluminium20 to 23% niobium 9 to 15% silicon 0.5 to 1.0% zirconium 1 to 3%titanium balance save for incidental impurities.
 5. A titanium alloy asclaimed in claim 1 consisting essentially of the following ingredientsin the atomic proportions below-stated:aluminum 20 to 23% niobium 9 to15% silicon 0.5 to 1.0% vanadium 1 to 3% titanium balance save forincidental impurities.
 6. A titanium alloy as claimed in claim 1consisting essentially of the following ingredients in the atomicproportions below-stated:aluminium 20 to 23% niobium 9 to 15% silicon0.5 to 1.0% molybdenum 1 to 3% titanium balance save for incidentalimpurities.
 7. A titanium alloy as claimed in claim 2 consistingessentially of the following ingredients in the atomic proportionsbelow-stated:aluminium 20-23% niobium approximately 11% siliconapproximately 0.9% titanium balance save for incidental impurities.titanium balance save for incidental impurities.
 8. A titanium alloy asclaimed in claim 4 consisting essentially of the following ingredientsin the atomic proportions below-stated:aluminium 20 to 23% niobiumapproximately 9% silicon 0.5 to 1.0% zirconium approximately 2% titaniumbalance save for incidental impurities.
 9. A titanium alloy as claimedin claim 5 consisting essentially of the following ingredients in theatomic proportions below-stated:aluminium 20 to 23% niobiumapproximately 9% silicon 0.5 to 1.0% vanadium approximately 2%titanium/balance save for incidental impurities.
 10. A titanium alloy asclaimed in claim 6 consisting essentially of the following ingredientsin the atomic proportions below-stated:aluminium 20 to 23% niobiumapproximately 9% silicon 0.5 to 1.0% molybdenum approximately 2%titanium balance save for incidental impurities.